This invention relates to methods and apparatus for making radiological measurements of human physiological properties and more particularly relates to improved radiological methods and apparatus for measuring changes in bone tissue in a living human being.
It is well known that the bone mineral content is an important physiological characteristic with respect to the health and well-being of a person, and therefore measurements of bone mineral content are often relied upon for diagnostic purposes. The usefulness of this diagnostic technique is, however, limited by the fact that present in vivos measurements do not accurately indicate relatively small changes, and by the fact that a more accurate measurement can only be obtained by invading the body for the purpose of removing an actual sample of the person's bone tissue.
There is described in the October 11, 1963, issue of Science, pp. 230-232, a technique for traversing the subject person with a beam of monoenergetic radiation for the purpose of determining bone mineral content. Although this technique has many useful applications, and although it has the advantage of not requiring an actual invasion of the human body to obtain a sample, it nevertheless suffers from limitations with respect to accuracy. The reason for this limitation is that any section of the human body containing bone tissue will also contain substantial quantities of other soft tissues. These tissues will, of course, have a different attenuation coefficient with respect to the bombarding radiation in the beam, and thus the resulting density measurement is actually a determination of the combined density of both bone and soft tissue. Moreover, this limitation is further complicated by the fact that the soft tissues in the target section are not all of the same density, since adipose tissue tends to have only about 80% (for a specific set of X-ray energies) of the attenuating capability of that exercised by other soft tissue. Consequently, measurements based on a determination of attenuation as practiced by the foregoing technique may provide error as great as 8% of the bone mineral content sought to be measured.
Although the term "density" is commonly used in connection with radiology, it is nevertheless somewhat imprecise. If the portion of the tissue irradiated by this technique is removed and ashed, the mass of such ash will correlate closely with the measurement provided with this technique. Similarly, it will also correlate with the mass of the bone. As used herein, therefore, the term bone mineral "density" shall mean the same as the terms bone mineral "mass" or bone mineral "content," inasmuch as no other term has achieved acceptance by those having experience in this art.
These disadvantages of the prior art are overcome with the present invention, and novel measurement techniques and apparatus are hereinafter described which employ polyenergetic radiation for the purpose of establishing two different but correlative measurements of the attenuating effect had by the target material, and wherein bone mineral content may be obtained from computations employing both such measurements together with preselected factors relating to source energy and the like.